Magnetic field strength detectors are known for measuring the strength of magnetic fields and generating one or more alarm signals in the event that the field strength exceeds a predetermined threshold. For example U.S. Pat. No. 3,604,373 entitled SECURITY STORAGE APPARATUS issued on Sept. 14, 1971, discloses a storage enclosure cabinet for storing magnetic tapes, discs, etc., provided with magnetic field detectors for signalling an excessive level of magnetism within the enclosure cabinet through audio and visual alarms placed at strategic remote points. According to the patented system, Hall Effect magnetic detection elements are provided in pairs for each of a plurality of respective storage spaces. The Hall Effect detectors are provided in pairs for the purpose of redundancy in the event that one of the detectors should fail.
Additional magnetic field strength detectors are disclosed in U.S. Pat. No. 3,961,320; 4,285,158; 4,296,376; 4,362,992; 2,551,596; and 4,527,123.